Mast cells (MCs) respond to activation by innate stimuli or cross linking of the high-affinity receptor for IgE (FceRI) by generating eicosanoids, particularly the cysteinyl leukotrienes (cysLTs) and prostaglandin (PG) D2, providing a direct link to the inflammatory processes in bronchial asthma. In addition to its ability to activate MC, our preliminary studies have now revealed that peptidoglycan (PGN), an innate immune stimulus which activates toll-like receptor (TLR)-2, can also induce the expression of LTC4 synthase (LTC4S) by mouse bone marrow-derived MCs (mBMMCs). Priming of mBMMCs with PGN increases their capacity to generate LTC4 upon ionophore stimulation and upon cross-linking of FceRI. LTC4S expression is also upregulated by an adaptive immune stimulus, interleukin (IL)-4. Moreover, the effect of PGN priming on LTC4S expression is additively/synergistically increased in the presence of IL-4. Because cysLTs play an important role in the pathogenesis of bronchial asthma, our results may explain the observed exacerbation of asthma by microbial or viral infection. Because TLR activates NF-?B and IL-4 activates signal transducer activator of transcription (STAT) 6, we hypothesize that 1) there is cooperation between the transcription factors STAT6 and NF-?B in regulating the transcription of LTC4S gene through the overlapping STAT6/NF-?B site(s), and 2) that activation of innate immune system by microbial or viral infections by TLR signaling pathways, aggravates bronchial asthma through an increase in cellular LTC4S expression above the maximal effect of adaptive immune activation by IL-4. We therefore propose the following Specific Aims: 1) To examine the effect of various TLR agonists on their ability to increase LTC4S expression, their effect in combination with IL-4 and the role of NF-?B transcription factor;2) To determine the mechanism of additive enhancement of LTC4S expression by IL-4 and PGN priming of mBMMC;and 3) To elucidate synergistic effects of IL-4 and TLR signaling in vivo on the pulmonary inflammatory response to antigen challenge and on airway reactivity to methacholine in surrogate mouse models of allergen induced airway disease.